Whether you need to clear security for a connecting flight depends on the type of connection and the airport layout. For a domestic-to-domestic flight on a single itinerary, you usually stay "airside" and do not need to re-clear security. However, if you are arriving on an international flight and connecting to a domestic one (especially in the U.S.), you must always clear customs, collect your bags, and go through security again before your next flight. Additionally, some airports (like Heathrow or LAX) have disconnected terminals that require you to exit the secure zone and re-enter security via a shuttle or walkway. If you booked two separate tickets (self-transfer), you will almost always have to clear security regardless of whether the flight is domestic or international.